This disclosure is directed to a circuit which varies the speed of a small motor. It is particularly intended for use with ceiling fans to vary the motor speed and hence the rate that air is moved by the ceiling fan. A ceiling fan is ordinarily operated at discrete speeds. The operation of a ceiling fan usually involves switching off and on with a main power switch. Many ceiling fans remain installed for many years of operation where the switch additionally includes a large and rather bulky series rheostat. It provides an adjustable voltage to the ceiling fan which changes the motor speed.
The present apparatus is intended to provide a retrofit to this type of equipment and particularly eliminates the large rheostat. In general terms, a rheostat is an energy wasting device. Further, it is a device which has some hazard in operation. It requires a contact to sweep across the coils thereby creating a potential hot spot and further creating a potentially electrically noisy operation. A ceiling fan typically is left in a particular condition for hours. That is, it is set at a particular speed and permitted to operate at that speed for many hours. The present apparatus is a switching system which places a series impedance in the circuit to thereby decrease the terminal voltage at the fractional horsepower motor and decrease motor speed.
It is well known to utilize starting capacitors with motors to control operation. Also, small motors are often constructed with starting circuits which drop out once speed has been achieved. It is, however, difficult to switch capacitors into a circuit for powering a motor without requiring a mechanical make/break switch via a relay or an on/off switch. The motor reflects an inductive load, and this will tend to suppress initial peak transients. When switching a capacitor into an electric motor circuit, there is always the risk of transient generation where the transient is of either polarity and of such amplitude that it may weld the contacts of a relay together or eventually the contacts will wear out. Mechanical contacts are not as reliable as solid state devices. Transient amplitude can easily be five or ten times greater than the nominal peak voltage. In light of this risk, switched series capacitors for controlling speed of a fan motor have not been desirable in the past.
Prior art devices include various and sundry motor controllers. For instance, U.S. Pat. No. 3,896,355 is a series triac motor control circuit. However, it does not incorporate series capacitors, and moreover, it shows a motor controller where the triacs are connected to different motor windings. It is similar to the IBM TDB Volume 9, No. 8 which shows series triacs again connected to different coils of a motor. U.S. Pat. No. 4,751,451 sets out a manually controlled variable capacitor switching approach, the manual control being implemented by hand insertion of a prepackaged pin equipped device into a socket. U.S. Pat. No. 4,670,699 is a similar hand controlled capacitor switching device. U.S. Pat. No. 4,352,993 is another hand switched type device. U.S. Pat. No. 4,007,378 shows the general idea of replacing a relay with a triac. U.S. Pat. No. 3,940,634 includes a triac which is serially connected. A relatively complex control system is provided for that. U.S. Pat. No. 3,737,761 shows a triac and capacitor in parallel so that the triac defeats the capacitor. It is connected also with a triac control circuit.
In general, the references mentioned above do not set forth a fractional horsepower fan motor control circuit of the sort found in the present disclosure and as described in the claims appended to the present disclosure.
The present disclosure is directed, however, to a switching circuit arranged in series with the fractional horsepower fan motor, there being duplicate circuits each providing similar but different sized capacitors. Each capacitor is switched into and out of the circuit by a triac in series therewith. The several triacs are each provided with control signals through an optical coupler. The optical coupler is provided with a control signal. Thus, with two capacitors, four different values of capacitance can be provided to provide four different speeds for the motor, one speed switching the motor off. If three different capacitors are used, eight speeds can be provided and if four capacitors are used, sixteen speeds can be provided.
Accordingly, the triac control of a series capacitor under the direction of an optical coupling circuit provides a neatly packaged retrofit device for incorporation in a motor controlled system which avoids the rheostat and yet provides variable speeds. Another alternate use of the present apparatus is as original equipment in a motor control circuit. The control signal which is provided is a low voltage switching signal, typically a DC level. It provides a continuous on/off signal to the triac and, therefore, is able to accomplish the desired control without contact closure where the contacts are required to handle the motor current. Also, the control system can be turned on/off from a remote controller by turning the triac(s) gate signal on or off.